OK, I hooked up a vacuum gauge and went for a ride. Vacuum needle was steady until fully warm and then rapidly vibrated +or- 1/2 inHg when the exhaust noise and vibration began at 55mph. I tried pulling the plug wires one at a time and it seemed like #2 made the least difference when pulled. But not a huge difference on any of them. I tried spraying starting fluid around the intake ports and carburetor base while idling and didn't notice any increase in rpm. Engine idle when fully warm fluctuates from 14 down to 13 on the vacuum gauge and I can hear a correlation between loss of vacuum on the needle and exhaust pipe noise volume increase. Exhaust pipe sucks up a piece of paper when engine misses. Overall, the truck ran better today than it has before disconnecting the manifold heat spring.
Does this sound like a finicky #2 exhaust valve?

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